1. Field of the Invention
This invention relates generally to apparatus and methods for object optimized rendering of computer generated page images into scan line raster data for driving a print engine.
2. Related Art
Prior to the advent of high quality computer generated page images, page images such as those found in newspapers, newsletters, magazines and the like were formed by graphic artists composing the page images by hand. Thus, each different type of object on a page image, including text, photographs, constant color areas or graphs such as pie charts, and sampled or continuously changing images such as sweeps, was optimally formed, independently of any other type of object.
Because these page images, comprising one or more of these types of objects, were composed by hand, each type of object was inherently treated independently of the other objects. Thus, the optimal halftone screen design for photographs, which differs from the optimal halftone screen designs for constant color areas and text, could be optimally selected and the screen arranged to an optimal angle. Likewise, such optimal treatment of each type of object can be obtained.
With the advent of digital color workstations, copiers and printers, creators of page images who would previously have had to rely on graphic artists to compose and print their page images could instead create, compose and print them on their own using a computer connected to the digital color copier/printer.
However, such prior art digital systems for creating a page image, for decomposing the page image into print engine instructions, and for controlling the print engine treated a page image as a single, unitary image. Thus, in the page image shown in FIG. 1A (which is optimized for text), when a high frequency halftone screen is used, the text portion of the page image is quite sharp. However, the large square constant color portion of the page image contains an obvious mottling from printer noise. In addition, the sampled color portion and the sweep portion of the page image show obvious contouring due to the lack of sufficient gray levels available with the high frequency screen.
In the page image shown in FIG. 1B (which is optimized for the large constant color portion), a halftone screen specifically designed to hide printer instabilities produces a high-quality, text- and artifact-free constant color area. However, the sharpness of the text is decreased and the gray values for each tint are not well-related, so that the sampled color portion and the sweep portion are unacceptable. The sweep portion demonstrates that the gray levels do not step smoothly from one to the next, because each dot level is designed separately without relation to the other levels.
In the page image shown in FIG. 1C (which is optimized for sampled color and sweep portions), the sweep portion and the sampled color portion show higher quality because a low frequency halftone screen is used, with more gray levels available. However, the text is shown in low quality and the constant color portion shows an obvious texturing.
Accordingly, as shown in FIGS. 1A-1C, in the prior art systems, which treated each page image as a single bitmap or bytemap image, optimizing the image for any one type of object required the image quality of the other types of objects to be compromised. This is also shown in FIGS. 1D and 1E, which show a radial sweep with text superimposed on top of the radial sweep. In FIG. 1D, both the text and the background sweep are printed using a high frequency halftone screen. While the text in FIG. 1D is shown in high quality, the background sweep has an obvious contouring artifact due to the low number of gray levels. In FIG. 1E, both the background sweep and the text are printed using a low frequency screen. While the background sweep is shown in high quality and without the contouring visible in FIG. 1D, the text is shown in very low quality and is essentially unreadable.
Accordingly, there is a need in the art for a digital color copier/printer and method for creating, decomposing and outputting a page image to a print engine which allows for the printing characteristics of individual objects to be optimized, as in the hand-composing graphic arts field, while simultaneously retaining the benefits and efficiencies available when creating a page image using a microcomputer.
Such page images are created using Page Description Languages (PDLs) such as PostScript.TM., Interpress.TM., Graphical Display Interfaces (GDIs), such as the one used with Windows.TM., Printer Command Languages (PCLs) for controlling a print engine, such as the Hewlett-Packard Printer Command Language (PCL-5).TM., or the like.